1. Field of the Invention
The present invention relates to optical apparatus, particularly focus detection and control systems for use with optical information-bearing-signal recorders.
2. Discussion of the Prior Art
Optical information-bearing-signal recorders, particularly of the optical or video disk type, retentively store information-bearing signals at extremely high areal densities. As such densities are increased, the preciseness required in controlling the recorder for faithful recording and reproduction of information-bearing signals greatly increases. One critical aspect of recording is to maintain the laser writing beam and the laser reading beam in focus at the appropriate level of the optical record medium. Various optical focusing systems have been provided, including those of the so-called "knife-edge" type. One advantage of knife-edge focus detectors is their simplicity. An example of such a detector is found in U.S. Pat. No. 4,079,248. While such known knife-edge focusing apparatus may be quite satisfactory for many present-day optical-disk areal densities, increased areal densities require yet greater focusing preciseness.
U.S. Pat. No. 4,517,666 (priority Japan, June 22, 1981, SN 56-96198) shows an optical focusing system having a light-shielding plate with an aperture, slit or knife edge arranged between the projection lens and the objective lens. Only part of the laser beam which passes through an area spaced apart from a common axis of the objective and projection lenses is transmitted through its aperture. While this arrangement provides a spatial disturbance of the light beam for focus detection, it does not show any mechanism for controlling the rate of change of focus-error signal at and near point of focus. This latter control is desired for facilitating focus serving at the point of focus at low cost.
For present-day optical information-bearing-signal storage devices, a focus-control circuit must keep the laser beam in focus with respect to the recording surface. Techniques which generate a signal indicative of a focus error rely on the fact that, when the recording surface is in focus and the focusing lens is illuminated with a collimated beam such as a laser beam, the reflected beam also is collimated. When the beam is out of focus, the returned, or reflected, beam is either converging or diverging, respectively, for the medium being too far from or too close to the optical system. The knife-edge detector, on the other hand, introduces asymmetry which appears at the detector only in out-of-focus conditions.
It is desired to enhance the dynamic range of the focus-error-signal generation beyond that provided by a regular knife-edge focus-error detector. Such dynamic range again provides for a greater preciseness in focus control which is useful at higher areal densities than presently employed.